A universal mobile telecommunication system (UMTS) is a European-type, third generation IMT-2000 mobile communication system that has evolved from a European standard known as Global System for Mobile communications (GSM). UMTS is intended to provide an improved mobile communication service based upon a GSM core network and wideband code division multiple access (W-CDMA) wireless connection technology. In December 1998, a Third Generation Partnership Project (3GPP) was formed by the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea. The 3GPP creates detailed specifications of UMTS technology.
In order to achieve rapid and efficient technical development of the UMTS, five technical specification groups (TSG) have been created within the 3GPP for standardizing the UMTS by considering the independent nature of the network elements and their operations. Each TSG develops, approves, and manages the standard specification within a related region. The radio access network (RAN) group (TSG-RAN) develops the standards for the functions, requirements, and interface of the UMTS terrestrial radio access network (UTRAN), which is a new radio access network for supporting W-CDMA access technology in the UMTS.
FIG. 1 provides an overview of a UMTS network. The UMTS network includes a mobile terminal or user equipment (UE) 1, a UTRAN 2 and a core network (CN) 3.
The UTRAN 2 includes several radio network controllers (RNCs) 4 and NodeBs 5 that are connected via the Iub interface. Each RNC 4 controls several NodeBs 5. Each NodeB 5 controls one or several cells, where a cell covers a given geographical area on a given frequency.
Each RNC 4 is connected via the Iu interface to the CN 3 or towards the mobile switching center (MSC) 6 entity of the CN and the general packet radio service (GPRS) support Node (SGSN) 7 entity. RNCs 4 can be connected to other RNCs via the Iur interface. The RNC 4 handles the assignment and management of radio resources and operates as an access point with respect to the CN 3.
The NodeBs 5 receive information sent by the physical layer of the UE 1 via an uplink and transmit data to the UE 1 via a downlink. The Node-Bs 5 operate as access points of the UTRAN 2 for the UE 1.
The SGSN 7 is connected to the equipment identity register (EIR) 8 via the Gf interface, to the MSC 6 via the Gs interface, to the gateway GPRS support node (GGSN) 9 via the GN interface, and to the home subscriber server (HSS) via the GR interface.
The EIR 8 hosts lists of UEs 1 that are allowed to use the network. The EIR 8 also hosts lists of UEs 1 that are not allowed to use on the network.
The MSC 6, which controls the connection for circuit switched (CS) services, is connected towards the media gateway (MGW) 11 via the Ng interface, towards the EIR 8 via the F interface, and towards the HSS 10 via the D interface.
The MGW 11 is connected towards the HSS 10 via the C interface and also to the public switched telephone network (PSTN). The MGW 11 also allows the codecs to adapt between the PSTN and the connected RAN.
The GGSN 9 is connected to the HSS 10 via the GC interface and to the Internet via the G1 interface. The GGSN 9 is responsible for routing, charging and separation of data flows into different radio access bearers (RABs). The HSS 10 handles the subscription data of users.
The UTRAN 2 constructs and maintains an RAB for communication between a UE 1 and the CN 3. The CN 3 requests end-to-end quality of service (QoS) requirements from the RAB and the RAB supports the QoS requirements set by the CN 3. Accordingly, the UTRAN 2 can satisfy the end-to-end QoS requirements by constructing and maintaining the RAB.
The services provided to a specific UE 1 are roughly divided into CS services and packet switched (PS) services. For example, a general voice conversation service is a CS service and a Web browsing service via an Internet connection is classified as a PS service.
The RNCs 4 are connected to the MSC 6 of the CN 3 and the MSC is connected to the gateway MSC (GMSC) that manages the connection with other networks in order to support CS services. The RNCs 4 are connected to the SGSN 7 and the gateway GGSN 9 of the CN 3 to support PS services.
The SGSN 7 supports packet communications with the RNCs. The GGSN 9 manages the connection with other packet switched networks, such as the Internet.
FIG. 2 illustrates a structure of a radio interface protocol between a UE 1 and the UTRAN 2 according to the 3GPP radio access network standards. As illustrated In FIG. 2, the radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control information. The U-plane is a region that handles traffic information with the user, such as voice or Internet protocol (IP) packets. The C-plane is a region that handles control information for an interface with a network as well as maintenance and management of a call. The protocol layers can be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on the three lower layers of an open system interconnection (OSI) standard model.
The first layer (L1), or physical layer, provides an information transfer service to an upper layer by using various radio transmission techniques. The physical layer is connected to an upper layer, or medium access control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel.
The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer. The MAC layer handles mapping between logical channels and transport channels and provides allocation of the MAC parameters for allocation and re-allocation of radio resources. The MAC layer is connected to an upper layer, or the radio link control (RLC) layer, via a logical channel.
Various logical channels are provided according to the type of information transmitted. A control channel is generally used to transmit information of the C-plane and a traffic channel is used to transmit information of the U-plane. A logical channel may be a common channel or a dedicated channel depending on whether the logical channel is shared.
FIG. 3 illustrates the different logical channels that exist. Logical channels include a dedicated traffic channel (DTCH), a dedicated control channel (DCCH), a common traffic channel (CTCH), a common control channel (CCCH), a broadcast control channel (BCCH), and a paging control channel (PCCH), or a Shared Control Channel (SCCH), as well as other channels. The BCCH provides information including information utilized by a UE 1 to access a system. The PCCH is used by the UTRAN 2 to access a UE 1.
Additional traffic and control channels are introduced in the Multimedia Broadcast Multicast Service (MBMS) standard for the purposes of MBMS. The MBMS point-to-multipoint control channel (MCCH) is used for transmission of MBMS control information. The MBMS point-to-multipoint traffic channel (MTCH) is used for transmitting MBMS service data. The MBMS scheduling channel (MSCH) is used to transmit scheduling information.
The MAC layer is connected to the physical layer by transport channels. The MAC layer can be divided into a MAC-b sub-layer, a MAC-d sub-layer, a MAC-c/sh sub-layer, a MAC-hs sub-layer and a MAC-m sublayer according to the type of transport channel being managed.
The MAC-b sub-layer manages a broadcast channel (BCH), which is a transport channel handling the broadcasting of system information. The MAC-c/sh sub-layer manages a common transport channel, such as a forward access channel (FACH) or a downlink shared channel (DSCH), which is shared by a plurality of UEs 1, or in the uplink the radio access channel (RACH). The MAC-m sublayer may handle MBMS data.
FIG. 4 illustrates the possible mapping between the logical channels and the transport channels from a UE 1 perspective. FIG. 5 illustrates the possible mapping between the logical channels and the transport channels from a UTRAN 2 perspective.
The MAC-d sub-layer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific UE 1. The MAC-d sublayer is located in a serving RNC 4 (SRNC) that manages a corresponding UE 1. One MAC-d sublayer also exists in each UE 1.
The RLC layer supports reliable data transmissions and performs segmentation and concatenation on a plurality of RLC service data units (SDUs) delivered from an upper layer depending of the RLC mode of operation. The RLC layer adjusts the size of each RLC SDU received from the upper layer in an appropriate manner based upon processing capacity and then creates data units by adding header information. The data units, or protocol data units (PDUs), are transferred to the MAC layer via a logical channel. The RLC layer includes a RLC buffer for storing the RLC SDUs and/or the RLC PDUs.
The BMC layer schedules a cell broadcast (CB) message transferred from the CN 3. The BMC layer broadcasts the CB message to UEs 1 positioned in a specific cell or cells.
The PDCP layer is located above the RLC layer. The PDCP layer is used to transmit network protocol data, such as the IPv4 or IPv6, efficiently on a radio interface with a relatively small bandwidth. The PDCP layer reduces unnecessary control information used in a wired network, a function called header compression, for this purpose.
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the C-plane. The RRC layer controls the transport channels and the physical channels in relation to setup, reconfiguration, and the release or cancellation of the radio bearers (RBs).
A RB signifies a service provided by the second layer (L2) for data transmission between a UE 1 and the UTRAN 2. The set up of the RB generally refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service and setting the respective detailed parameters and operation methods. The RRC also handles user mobility within the RAN and additional services, such as location services.
Not all different possibilities for the mapping between the RBs and the transport channels for a given UE 1 are available all the time. The UE 1/UTRAN 2 deduce the possible mapping depending on the UE state and the procedure presently executed by the UE/UTRAN.
The different transport channels are mapped onto different physical channels. The configuration of the physical channels is given by RRC signaling exchanged between the RNC 4 and the UE 1.
Initial access is a procedure whereby a UE 1 sends a first message to the UTRAN 2 using a common uplink channel, specifically the Random Access Channel (RACH). For both GSM and UMTS systems, the initial access procedure involves the UE 1 transmitting a connection request message that includes a reason for the request and receiving a response from the UTRAN 2 indicating the allocation of radio resources for the requested reason.
There are several reasons, or establishment causes, for sending a connection request message. Table 1 indicates the establishment causes specified in UMTS, specifically in 3GPP TS 25.331.
TABLE 1Establishment CausesOriginating Conversational CallOriginating Streaming CallOriginating Interactive CallOriginating Background CallOriginating Subscribed traffic CallTerminating Conversational CallTerminating Streaming CallTerminating Interactive CallTerminating Background CallEmergency CallInter-RAT cell re-selectionInter-RAT cell change orderRegistrationDetachOriginating High Priority SignalingOriginating Low Priority SignalingCall re-establishmentTerminating High Priority SignalingTerminating Low Priority Signaling
The “Originating call” establishment cause indicates that the UE 1 wants to setup a connection, for example, a speech connection. The “terminating call” establishment cause indicates that that UE 1 answers to paging. The “registration” establishment cause indicates that that the user wants to register only to the network.
A physical random access procedure is used to send information over the air. The physical random access transmission is under control of a higher layer protocol, which performs important functions related to priority and load control. This procedure differs between GSM and UMTS radio systems.
As the present invention is related to UMTS enhancement and evolution, the W-CDMA random access procedure is detailed herein. Although the present invention is explained in the context of UMTS evolution, the present invention is not so limited.
The transport channel RACH and two physical channels, Physical Random Access Channel (PRACH) and Acquisition Indication Channel (AICH), are utilized in this procedure. The transport channels are channels supplied by the physical layer to the protocol layer of the MAC layer. There are several types of transport channels to transmit data with different properties and transmission formats over the physical layer.
Physical channels are identified by code and frequency in Frequency Division Duplex (FDD) mode and are generally based on a layer configuration of radio frames and timeslots. The form of radio frames and timeslots depends on the symbol rate of the physical channel.
A radio frame is the minimum unit in the decoding process, consisting of 15 time slots. A time slot is the minimum unit in the Layer 1 bit sequence. Therefore, the number of bits that can be accommodated in one time slot depends on the physical channel.
The transport channel RACH is an uplink common channel used for transmitting control information and user data. The transport channel RACH is utilized in random access and used for low-rate data transmissions from a higher layer. The RACH is mapped to an uplink physical channel, specifically the PRACH. The AICH is a downlink common channel, which exists as a pair with PRACH used for random access control.
The transmission of PRACH is based on a slotted ALOHA approach with fast acquisition indication. The UE randomly selects an access resource and transmits a RACH preamble part of a random access procedure to the network.
A preamble is a short signal that is sent before the transmission of the RACH connection request message. The UE 1 repeatedly transmits the preamble by increasing the transmission power each time the preamble is sent until it receives the Acquisition Indicator (AI) on AICH, which indicates the detection of the preamble by the UTRAN 2. The UE 1 stops the transmission of the preamble once it receives the AI and sends the message part at the power level equal to the preamble transmission power at that point, adding an offset signaled by the UTRAN 2. FIG. 6 illustrates a power ramping procedure.
This random access procedure avoids a power ramping procedure for the entire message. A power ramping procedure would create more interference due to unsuccessfully sent messages and would be less efficient due to a larger delay since it would take much more time to decode the message before an acknowledgement could be transmitted to indicate successful receipt of the message.
The main characteristics of the RACH is that it is a contention based channel subject to collisions due to simultaneous access of several users, which may preclude decoding of the initial access message by the network. The UE 1 can start the random access transmission of both preambles and message only at the beginning of an access slot. This access method is, therefore, a type of slotted ALOHA approach with fast acquisition indication.
The time axis of both the RACH and the AICH is divided into time intervals or access slots. There are 15 access slots per two frames, with each frame having a length of 10 ms or 38400 chips, and the access slots are spaced 1.33 ms or 5120 chips apart. FIG. 7 illustrates the number and spacing of access slots.
The UTRAN 2 signals information regarding which access slots are available for random access transmission and the timing offsets to use between RACH and AICH, between two successive preambles and between the last preamble and the message. For example, if the AICH transmission timing is 0 and 1, it is sent three and four access slots after the last preamble access slot transmitted, respectively. FIG. 8 illustrates the timing of the preamble, AI and message part.
The timing at which the UE 1 can send the preamble is divided by random access sub channels. A random access sub channel is a subset including the combination of all uplink access slots. There are 12 random access sub channels. A random access sub channel consists of the access slots indicated in Table 2.
TABLE 2SFN modulo8 ofcorrespondingP-CCPCHSub-channel numberframe012345678910110012345671121314891011201234567391011121314846701234558910111213146345670127891011121314
The preamble is a short signal that is sent before the transmission of the RACH message. A preamble consists of 4096 chips, which is a sequence of 256 repetitions of Hadamard codes of length 16 and scrambling codes assigned from the upper layer.
The Hadamard codes are referred to as the signature of the preamble. There are 16 different signatures and a signature is randomly selected from available signature sets on the basis of Access Service Classes (ASC) and repeated 256 times for each transmission of the preamble part. Table 3 lists the preamble signatures.
The message part is spread by Orthogonal Variable Spreading Factor (OVSF) codes that are uniquely defined by the preamble signature and the spreading codes for use as the preamble signature. The 10 ms long message part radio frame is divided into 15 slots, each slot consisting of 2560 chips.
Each slot includes a data part and a control part that transmits control information, such as pilot bits and TFCI. The data part and the control part are transmitted in parallel. The 20 ms long message part consists of two consecutive message part radio frames. The data part consists of 10*2 k bits, where k=0, 1, 2, 3, which corresponds to a Spreading Factor (SF) of 256, 128, 64, 32. FIG. 9 illustrates the structure of the random access message part.
TABLE 3PreambleValue of nsignature0123456789101112131415P0(n)1111111111111111P1(n)1−11−11−11−11−11−11−11−1P2(n)11−1−111−1−111−1−111−1−1P3(n)1−1−111−1−111−1−111−1−11P4(n)1111−1−1−1−11111−1−1−1−1P5(n)1−11−1−11−111−11−1−11−11P6(n)11−1−1−1−11111−1−1−1−111P7(n)1−1−11−111−11−1−11−111−1P8(n)11111111−1−1−1−1−1−1−1−1P9(n)1−11−11−11−1−11−11−11−11P10(n)11−1−111−1−1−1−111−1−111P11(n)1−1−111−1−11−111−1−111−1P12(n)1111−1−1−1−1−1−1−1−11111P13(n)1−11−1−11−11−11−111−11−1P14(n)11−1−1−1−111−1−11111−1−1P15(n)1−1−11−111−1−111−11−1−11
The AICH consists of a repeated sequence of 15 consecutive access slots, each slot having a length of 40 bit intervals or 5120 chips. Each access slot includes two parts, an Acquisition Indicator (AI) part consisting of 32 real-valued signals, such as a0 ?a31, and a part having a length of 1024 chips during which transmission is switched off. FIG. 10 illustrates the structure of the AICH.
When the UTRAN 2 detects transmission of a RACH preamble having a certain signature in an RACH access slot, the UTRAN repeats this signature in the associated AICH access slot. Therefore, the Hadamard code used as the signature for the RACH preamble is modulated onto the AI part of the AICH.
The acquisition indicator corresponding to a signature can have a value of +1, −1 or 0 depending on whether a positive acknowledgement (ACK), a negative acknowledgement (NACK) or no acknowledgement is received in response to a specific signature. The positive polarity of the signature indicates that the preamble has been acquired and the message can be sent.
The negative polarity indicates that the preamble has been acquired and the power ramping procedure shall be stopped, but the message shall not be sent. This negative acknowledgement is used when a received preamble cannot be processed at the present time due to congestion in the UTRAN 2 and the UE 1 must repeat the access attempt some time later.
All UEs 1 are members of one of ten randomly allocated mobile populations, defined as Access Classes (AC) 0 to 9. The population number is stored in the Subscriber Identity Module (SIM)/Universal Subscriber Identity Module (USIM). UEs 1 may also be members of one or more out of 5 special categories of Access Classes 11 to 15, which are allocated to specific high priority users and the information also stored in the SIM/USIM. Table 4 lists the special AC and their allocation.
TABLE 4ACAllocation15PLMN Staff14Emergency Services13Public Utilities (e.g. water/gas suppliers)12Security Services11
The UTRAN 2 performs the random access procedure at protocol layer L2 by determining whether to permit the UE 1 to use a radio access resource based primarily upon the AC to which the UE belongs.
It will be desirable to prevent UE 1 users from making access attempts, including emergency call attempts, or responding to pages in specified areas of a Public Land Mobile Network (PLMN) under certain circumstances. Such situations may arise during states of emergency or where 1 or more co-located PLMNs has failed. Broadcast messages should be available on a cell-by-cell basis to indicate the class(es) of subscribers barred from network access. The use of this facility allows the network operator to prevent overload of the access channel under critical conditions.
Access attempts are allowed if the UE 1 is a member of at least one AC that corresponds to the permitted classes as signaled over the air interface and the AC is applicable in the serving UTRAN 2. Access attempts are otherwise not allowed. Any number of these AC may be barred at any one time. Access Classes are applicable as indicated in Table 5.
TABLE 5ACApplicability0-9Home and Visited PLMNs11 and 15Home PLMN only12, 13, 14Home PLMN and visited PLMNs of home country only
An additional control bit for AC 10 is also signaled over the air interface to the UE 1. This control bit indicates whether access to the UTRAN 2 is allowed for Emergency Calls for UEs 1 with access classes 0 to 9 or without an International Mobile Subscriber Identity (IMSI). Emergency calls are not allowed if both AC 10 and the relevant AC, 11 to 15 are barred for UEs 1 with access classes 11 to 15. Emergency calls are otherwise allowed.
The AC are mapped to ASC In the UMTS. There are eight different priority levels defined, specifically ASC 0 to ASC 7, with level 0 representing the highest priority.
Access Classes shall only be applied at initial access, such as when sending an RRC Connection Request message. A mapping between AC and ASC shall be indicated by the information element “AC-to-ASC mapping” in System Information Block type 5. The correspondence between AC and ASC is indicated in Table 6.
TABLE 6AC0 ?9101112131415ASC1st IE2nd IE3rd IE4th IE5th IE6th IE7th IE
In Table 6, “nth IE” designates an ASC number i in the range 0-7 to AC. The UE 1 behavior is unspecified if the ASC indicated by the “nth IE” is undefined.
The parameters implied by the respective ASC are utilized for random access. A UE 1 that is a member of several ACs selects the ASC for the highest AC number. The AC is not applied in connected mode.
An ASC consists of a subset of RACH preamble signatures and access slots that are allowed for the present access attempt and a persistence value corresponding to a probability, Pv≦1, to attempt a transmission. Another important mechanism to control random access transmission is a load control mechanism that reduces the load of incoming traffic when the collision probability is high or when the radio resources are low.
The UMTS random access procedure is illustrated in FIG. 11. Signatures are transmitted from the UE 1 to the NodeB 5 during a ramping cycle of the UMTS random access procedure until the NodeB sends an ACK or a NACK to the UE. The UE 1 sends the message to the Node-B 5 upon receiving an ACK corresponding to a transmitted signature.
The ramping cycle is repeated by the MAC layer up to a maximum allowed number of repetitions for this layer if the UE 1 receives a NACK or no answer from the Node-B 5. The entire procedure can be repeated by the RRC if no positive acknowledgement has been received after the maximum allowed number of repetitions by the MAC layer. A persistency value is attributed to a UE 1 that tries to access the RACH. The persistency value consists of a random time that the UE 1 must wait before transmitting a preamble. The random time is intended to resolve a potential overload and to reduce the likelihood that two UEs 1 that have started the ramping cycle simultaneously fail due to their mutual interference and restart the ramping cycle again at the same time. The ramping cycle corresponds to several preamble transmissions before the back-off procedure in the MAC layer and between each of these transmissions the power is incremented by the ramping cycle.
The persistency in UMTS is only used before the first preamble transmission of a ramping cycle. Therefore, there will not be any delay similar to the persistency value between two consecutive attempts if the UE 1 makes more than one attempt.
Two other cycles are distinguished in UMTS, each one allowing several random access attempts with several ramping cycles and introducing delay of related timers before each attempt. These cycles and the corresponding timers are handled by, respectively, the MAC and RRC layers:
The first cycle is a back-off procedure in the MAC layer when a ramping cycle is over or, in other words, the maximum number of preamble transmissions has been reached for the physical layer. The back-off procedure consists of a back-off timer, whose expiration time defines a time the UE 1 has to wait before restarting the ramping cycle, and a counter, which defines a maximum number of back-off procedures before the procedure transfers to the RRC level.
The second cycle is performed in the lower layers where a counter allows several random access attempts and a delay timer (T300) at the RRC level introduces additional delay. The UE 1 can no longer attempt random access and transitions to an IDLE mode when the counter reaches its maximum value. This cycle allows the RRC layer to take over and eventually stop the procedure, such as when the RRC receives new information.
FIG. 11 illustrates the elementary steps of the random access procedure, with a clear illustration of the number of random access attempts at the RRC, MAC and PHY layers. As illustrated in FIG. 11, a UE 1 accesses RACH (START) in order to transmit a message.
The UE 1 is allowed several random access attempts at different levels, specifically the RRC, MAC and physical layers. A counter will be incremented at each level to count the number of random access attempts. Each counter (V300, Mcurrent and Ncurrent) are set to an initial value of zero before the first random access attempt at the corresponding level.
The UE 1 retrieves RACH information on the BCH, such as available signatures for random access and power ramping information (S100). The UE 1 then randomly selects a signature (S102).
It is then determined whether a persistency value must be applied (S104). A persistency value is applied (S106) if necessary and the UE 1 must wait for the persistency timer to expire before continuing the random access procedure. The persistency value may be chosen randomly, but may still depend on the ASC or on the number of random access attempts the UE 1 has already performed (Ncurrent).
The random access procedure may be optimized by distinguishing two different situations according to the reason the UE 1 has initiated the random access procedure. The persistency test is applied even for the first attempt if the random access is related to TA update or counting or other requests related to MBMS. The persistency is not applied in all the other cases, such as for the first random access attempt at the PHY layer.
The UE 1 sends the PHY ACCESS REQ message to the Node-B 5 to request random access (S108). It is then determined if the random access was successful (S110) and the random access procedure is complete if the random access was successful. It is determined if the maximum number of random access attempts have been made at the PHY layer if the random access was not successful (S112).
Power ramping is applied by incrementing power by one step (S114), a PHY layer access counter (Ncurrent) is incremented (S116), and the random access procedure continues by determining whether a persistency value must be applied (S104) if the maximum number of allowed random access attempts at the PHY layer (Nmax) has not been reached. It is determined if the maximum number of random access attempts for the MAC layer has been reached (S118) by comparing a counter (Mcurrent) to the maximum number of allowed random access attempts at the MAC layer (Mmax) if the maximum number of random attempts for the physical layer has been reached.
The back-off procedure is applied if the maximum number of allowed random access attempts for the MAC layer has not been reached. A MAC access cycle counter (Mcurrent) is incremented, the PHY layer access counter (Ncurrent) is initialized (S122) and the UE 1 must wait for the back-off timer to expire (S124) before continuing the random access procedure by retrieving RACH information (S100). It is determined if the maximum number of random access attempts at the RRC layer has been reached (S126) by comparing an RRC access counter (V300) to the maximum number of allowed random access attempts at the RRC layer (N300) if the maximum number of allowed random access attempts for the MAC layer has been reached.
The UE transitions to IDLE mode if the maximum number of allowed random access attempts at the RRC layer (N300) has been reached. The UE 1 must wait for an RRC timer (T300) to expire (S128), increment the RRC access counter (V300) (S130), and initialize the MAC access cycle counter (Mcurrent) (S132) before continuing the random access procedure by retrieving RACH information (S100).
The long-term evolution of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS. The 3GPP Long-Term Evolution (LTE) project is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP initiated the LTE to ensure competitiveness of radio-access networks for ten years and beyond. LTE will not lead to a standard but to evolved releases of the UMTS standards. The goals of LTE are increased spectrum efficiency, lower costs, improved services and better integration with other standards. The requirements for data rates are indicated in Table 7.
FIG. 12 illustrates the architecture of an LTE system. Each aGW 115 is connected to one or several access Gateways (aGW) 115. An aGW 115 is connected to another node (not shown) that allows access to the Internet and/or other networks, such as GSM, UMTS, and WLAN.
TABLE 7UMTSNetworksEDGEUMTS(HS-DPA)LTETheoretical peak473.6 kbit/s 2 Mbit/s14.4 Mbit/s100 Mbit/sdata rateMeasured peak  180 kbit/s384 kbit/s 3.6 Mbit/s 50 Mbits/sdata rate
The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, simple structure, an open interface, and adequate power consumption of a UE 1 as an upper-level requirement. The UTRAN 2 generally corresponds to the E-UTRAN (Evolved-UTRAN) and the NodeB 5 and/or RNC 4 correspond to e-NodeB (eNB) 5 in the LTE system.
The LTE is a new air interface based on multicarrier on orthogonal frequency division multiplexing (OFDM) for downlink transmissions and based on single carrier DFT-S-OFDM (Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing) for uplink transmissions. The aGW 115 network is entirely optimized for only packet switched data and circuit switched data is not supported.
LTE provides a flexible spectrum management since the specifications are designed for multiple bandwidth allocations, specifically 1.4, 3, 5, 10, 15 and 20 MHz, whereas the former CDMA systems required 5 MHz band. Peak data rates, coverage, high-speed terminals and delay are particularly important.
A layer can be seen as a set of procedures that grant services for upper layers. The physical layer offers data transport services to higher layers. The radio interface is the interface between the UE 1 and the aGW 115. The radio interface is composed of layer1, layer2, and layer3. Each layer offers its own set of services, such as segmentation, in-sequence delivery, or error correction trough the use of automatic repeat request (ARQ) and hybrid automatic repeat request (HARQ).
FIG. 13 illustrates the radio interface architecture for LTE. The MAC and RRC layers are sub-layers of layer2. The arrows represent primitives or service access points. A flow of bits is called a channel. The channels between the MAC and RRC layers are logical channels and are defined by the type of information they carry. A general classification of logical channels can be made by separating the control channels used for transfer of control-plane information from the traffic channels used for transfer of user-plane information.
The channels between the PHY and MAC layers are transport channels characterized by the way information is transmitted over the air interface. The time and frequency resources used by a specific channel are called a physical channel.
The physical layer provides data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The functions performed by the physical layer in order to provide the data transport service are listed in Table 8.
TABLE 8Error detection on the transport channel and indication to higher layersEncoding/decoding of the transport channelHybrid ARQ soft-combiningRate matching of the coded transport channel to physical channelsMapping of the coded transport channel onto physical channelsPower weighting of physical channelsModulation and demodulation of physical channelsFrequency and time synchronizationRadio characteristics measurements and indication to higher layersMultiple Input Multiple Output (MIMO) antenna processingBeamformingRF processing
The physical layer receives and passes information from and to the MAC sub-layer. For example, the MAC scheduler indicates to the PHY layer the modulation scheme to use for transmitting data, such as QPSK, 16QAM, or 64QAM, and the channel quality indicators (CQI) are reported to the RRC layer.
Layer 1 is responsible for the transport of data, such as channel coding, segmentation, and scrambling codes, between a UE 1 and an eNB 105. Layer 2 controls resource assignment.
The procedures performed by the physical layer are Cell search, Power control, Uplink synchronization, Random access and HARQ. Through the control of physical layer resources in the frequency domain as well as in the time and power domain, implicit support of interference coordination is provided in LTE.
Cell search is the procedure by which a UE 1 selects an eNB 5 and a cell from among all detected eNB's. Power control is the procedure used to set the UE 1 transmitted power to the most appropriate value. Uplink synchronization is the procedure used to align the UE 1 local oscillator to the eNB 5 clock in the frequency domain. Random access is the procedure used by the UE 1 to obtain time synchronization with an eNB 5. HARQ is the procedure by which a receiver can acknowledge the correct reception of transport blocks from a transmitter.
Frequency and time resources are shared according to a multiple access scheme in mobile networks. This scheme is based on OFDM in the downlink and single-carrier frequency division multiple access (SC-FDMA) in the uplink. Both schemes use cyclic prefixes (CP) and will be further detailed later.
FIG. 14 illustrates the LTE frame structure. The basic unit of time is a slot with a duration fixed at 0.5 ms. A subframe is 1 ms and consists of two slots. A radio frame is 10 ms and consists of ten subframes. This frame structure is applicable to both Frequency division duplex (FDD) and Time Division Duplex (TDD).
Ten subframes are available for downlink transmission and ten subframes are available for uplink transmissions in each 10 ms interval for FDD. A subframe is either allocated to downlink or uplink transmission for TDD. Uplink and downlink transmissions are separated in the frequency domain.
A grid of resource elements describes the signals transmitted in each slot. The length and the bandwidth of a signal are given by the resource elements allocated to that signal.
The resource element is the smallest unit of resources in each slot and is defined by a pair of indexes (k, l). The index k=0, . . . , Nsc−1 indicates the subcarrier index within the system bandwidth and l=0, . . . , Nsymb−1 indicates the symbol index within the slot, where a ‘symbol’ refers to an OFDM symbol in downlink and a SC-FDMA (Single Carrier Frequency Division Multiple Access) symbol in uplink and Nsc and Nsymb are the number of subcarriers and the number of symbols available in a slot.
A resource block is the smallest unit of resources allocated to a signal. A resource block is defined as Nsymb consecutive symbols in the time domain and NscRB consecutive subcarriers in the frequency domain, where Nsymb and NscRB have the values listed in Table 9. Therefore, a resource block in the uplink consists of 0.5 ms in the time domain and 180 kHz in the frequency domain. The number of resource blocks in each slot depends on the system bandwidth. Table 10 illustrates NRB for different bandwidths. The resource elements and resource blocks are represented in the resource grid in FIG. 15(a) for uplink (UL) and FIG. 15(b) for downlink (DL).
TABLE 9ConfigurationNscRBNsymbNormal cyclic prefix127Extended cyclic prefix126
TABLE 10System90% efficiencybandwidthbandwidth(MHz)(MHz)NRB1.41.2663.02.71254.525109502018100
The random access channel (RACH) is the physical channel dedicated to the random access procedure for Layer 1. All uplink transmissions are initiated through the RACH. The RACH can be used for several purposes. The RACH function is different depending on the technology of the system. The RACH can be used to access the eNB 5, to request resources, to carry control information, to adjust the time offset of the uplink, or to adjust the transmitted power.
The RACH is considered contention-based because UEs 1 sending data on the RACH are not identified by the target eNB 5. Therefore, contention resolution is the major issue since many users may attempt to access the same base station simultaneously, thereby causing collisions.
The RACH occupies 6 resource blocks in a subframe or set of consecutive subframes reserved for random access preamble transmissions. The RACH period is not fixed. FIG. 16 illustrates one possible mapping of the RACH within the resource grid.
The LTE requirements for RACH are different than for UMTS. While the RACH is primarily used to register the UE 1 to the Node-B 5 after power-on in 3G systems, the LTE RACH is subject to different constraints.
The messages sent in an OFDM-based system are orthogonal. Therefore, the physical layer is designed differently. A major challenge in such a system is to maintain uplink orthogonality among UEs 1, which requires both frequency and time synchronization of the signals transmitted from the UEs.
Frequency synchronization can be achieved by fixing the transmitter local oscillator to the clock of the downlink broadcast signal. The remaining frequency misalignment at the eNB 5 is due to Doppler effects, which are neither estimated nor compensated and, therefore, require no further consideration.
However, the timing estimation has to be performed by the eNB 5 when measuring the received signal. This can be achieved during the random access procedure. The UE 1 then receives a timing advance command from the eNB 5 and adjusts its uplink transmission timing accordingly. Consequently, one purpose of the random access procedure is to obtain uplink time synchronization.
A UE 1 only has access to the slot and frame number in the downlink prior to random access. In other words, the UE 1 receives the start and the end of slots and frames from a broadcast signal but the transmission delay implies a time shift between the transmission and the reception of the broadcast signal. Therefore, the UE 1 cannot estimate when to send data such that the eNB 5 receives the data at the beginning of a slot. All the UEs 1 still must be synchronized with the cell base to avoid interference. FIG. 17 illustrates this propagation delay.
The random access procedure is the procedure by which a UE 1 obtains timing synchronization with an eNB 5. The UE 1 or the eNB 5 can initiate the random access procedure. The random access procedure is triggered by the events listed In Table 11.
TABLE 11Events Triggering Random Access ProcedureUE switches from power-off to power-on and needs to beregistered to the networkUE transmitting and not time-synchronized with eNB(i.e. user makes a call)eNB transmitting data to UE but they are not synchronized(i.e. user receives a call).eNB measures delay of received signal from UE(i.e. user moving and loses synchronization)UE moving from one cell to another and needs to be time-synchronized with different eNB than eNB to which it isregistered (i.e. handover).
The UE 1 selects and generates a single random access burst once the random access procedure is requested. This single random access burst is sent on the RACH with parameters derived from previous measurements on the downlink broadcast channel (BCH), such as frequency position, time period, and target power.
The random access burst consists of a cyclic prefix, a preamble, and a guard time during which nothing is transmitted as illustrated in FIG. 18. The preamble is chosen by the UE 1 from a set of signatures known by the eNB 5. A collision occurs whenever several UEs 1 choose the same signature.
The random access burst is transmitted during one subframe. A random access burst from a UE 1 that is not synchronized in the time domain can overlap with the next subframe and generate interference. Therefore, a guard time is necessary. The guard time (GT) must be at least equal to the round-trip delay at the cell edge.
For example, the maximum cell radius (R) supported by the burst of FIG. 19 is defined by the following equation, with a larger cell requiring a longer guard time:R=c·TGT/2≈15 km.
Several users share the same channel during the random access procedure and are distinguishable due to orthogonal sequences. The orthogonal sequences are seen as UE 1 signatures that can be transmitted simultaneously and must satisfy criteria, such as good autocorrelation properties for accurate timing estimation of a single preamble and good cross correlation properties for accurate timing estimation of different simultaneous preambles. Zadoff-Chu (ZC) sequences are used in 3GPP to fulfill these requirements.
Each cell possesses a set of 64 signatures obtained from Zadoff-Chu (ZC) sequences. The length of one sequence is N=839 samples. A ZC sequence is defined by two integers, ‘u’ as the root index and ‘v’?as the cyclic shift index.
The ‘v-th’ cyclic shift is extracted from the ‘u’-th?root in the time domain according to the following equation:xu,v(n)=xu(n+v·NCS),
where n=0 . . . N−1 and NCS is the cyclic shift length.
The ‘u-th’ root sequence in the frequency domain is defined by the following equation:Xu(n)=eiπ·u·(n(n+1)/N) 
The ZC sequences have been chosen because they can generate a large number of sequences and they offer correlation properties such that the autocorrelation function shows no side peaks. The cross correlation between two sequences obtained from different roots is vN. Therefore, ZC sequences have zero-cross-correlation zones.
The random access procedure may be contention-free such that a UE 1 sends a message on the RACH without collision with the message from another UE. This may happen during handover because the eNB 5 is able to allocate a reserved signature or code to a specific UE 1. These dedicated signatures are allocated by the eNB 5 only.
FIG. 20 illustrates the sequence of messages and responses exchanged between the UE 1 and the eNB 5. The random procedure is a five-step process.
First the UE 1 retrieves information using message 1 on the BCH. The information is related to available signatures in the cell, RACH slots location and period. The UE sets its transmit power according to the signal attenuation measured in the downlink, which is open-loop power control.
The UE 1 then selects one of the available slots and sends message 2. The second message is the random access burst containing the chosen signature.
The eNB 5 then tries to detect preambles during the current RACH slot and acknowledges the successfully detected preambles in message 3. Message 3 contains a timing advance command and a power-control command and is sent on a dedicated downlink channel using the detected signature.
The UE 1 and the eNB5 are now aligned in the time domain and the procedure ends if the procedure was contention-free. The procedure contains two more steps involving message 4 and message 5 if the procedure was not contention-free.
The UE 1 adjusts power and timing and sends a resource request message on a dedicated uplink channel if it has received an answer from the eNB 5. The UE 1 requests bandwidth and time resources in order to transmit data and also indicates a UE-specific identifier in message 4.
The UE 1 waits for the next RACH slot to send another preamble if no response corresponding to the transmitted preamble sequence is received. The procedure is terminated after a certain number of failures. The timing-advance command instructs the UE 1 to correct its transmission timing by a multiple of 0.52 ms, which is referred to as granularity.
The eNB 5 then resolves contentions. Either the UE 1 was in collision and message 5 provides the command to re-start the procedure or the UE was not in collision and the message 5 is a resource assignment with the next transmissions performed as usual.
The detailed random access procedure in the UE 1 procedure will now be described. The UE 1 listens to a downlink broadcast signal to receive information related to the available signatures, frequency bands, time slots, and power settings for a random access.
Open-loop power control can be used to obtain a suitable transmission power. The UE 1 estimates path loss from a downlink reference signal and sets the transmission power to achieve a signal-to-noise ratio (SNR) target indicated by the eNB 5.
The eNB 5 may fix the targeted SNR upon the measured level of uplink interference. The shadowing in the uplink path may differ from the shadowing in the downlink path because the carrier frequency has changed.
The UE 1 randomly selects a signature, a time slot and a frequency band from the available set. The UE 1 then sends a burst containing the chosen signature over the selected RACH slot.
The UE 1 decodes a received positive response and adapts its transmission timing. The UE 1 also adapts its transmission power if the response contains power control information. The UE 1 may request resources and use a specific identifier (ID) in the message to resolve contentions.
The UE 1 then monitors a specified downlink channel for a response from the eNB 5. The next transmissions are performed normally if a positive resource grant is received. The UE 1 restarts the random access procedure if a collision indicator is received or no response is received from the eNB 5.
A new random access attempt is performed in the next available RACH slot if the UE 1 does not receive a response from the eNB 5. The UE 1 should keep the same signature and the transmission power may be increased using a power ramping method.
The detailed random access procedure in the eNB 5 will now be described. The eNB 5 updates the information transmitted on the BCH periodically.
The eNB 5 monitors the RACH slot in expectation of random accesses. The eNB 5 correlates the received signal in the RACH sub-frame with all possible signatures. The detection can be performed either in the time domain or in the frequency domain using a process that will be described later.
A detection variable is computed for each signature. The signal is considered detected if the detection variable exceeds a certain threshold.
The timing offset is then computed from the peak position. The eNB 5 could also estimate a power adjustment from the values of the detection variables.
The eNB 5 sends a response using the detected signature. This acknowledgement is sent over dedicated resources.
The eNB 5 determines how many UEs 1 were detected with the same signature and resolves the possible contentions if a resource request with a UE-specific ID is received. The eNB 5 also identifies the UE 1 and assigns resources according to scheduling rules.
The UE 1 waits for the next RACH slot to re-send the preamble if a preamble is not detected in the first attempt. The preamble signal-to-noise ratio (SNR) is relatively low compared to the data SNR due to the length of the zero-correlation sequences. The UEs 1 can increase the transmit power by a few decibels (dB) for the second attempt in order to prevent consecutive failures since the random access channel does not generate much interference. A long delay is not desirable, especially for handover.